1. Field of the Invention
The present invention relates to an objective lens provided in the tip portion of an endoscope. More particularly, the invention relates to an endoscope objective lens capable of being applied to an optical system in which the back focal length is long enough to allow an optical path-changing prism to be disposed on the image side.
2. Description of Related Art
Endoscopes are known in which a solid-state imaging device such as a CCD (charge-coupled device) is provided in the tip portion so as to be parallel with the longitudinal direction. FIG. 17 outlines an objective optical system of such an endoscope. As shown in FIG. 17, a solid-state imaging device 200 is disposed in the tip portion of the endoscope so as to be parallel with the axial direction (longitudinal direction) Z. An optical path-changing prism 102 is inserted between an objective lens 100 and the solid-state imaging device 200 via a cover glass 101. The optical axis Z1 of the objective lens 100 is bent by the optical path-changing prism 102 by about 90° so as to be directed to the solid-state imaging device 200. The size of the optical path-changing prism 102 is determined by an image size, and the length of the space between the last surface of the objective lens 100 and an image forming position P where the optical path-changing prism 102 is to be inserted should be sufficiently long (this length is approximately equal to the back focal length). This requires an objective lens whose back focal length is long relative to the focal length of the whole system. However, to meet the wide angle requirement, the focal lengths of endoscope objective lenses tend to decrease even for the same image size, which makes it difficult to obtain a sufficiently long back focal length. In this connection, an endoscope objective lens having a 3-group/4-lens structure whose back focal length is sufficiently longer than the combined focal length f of the whole system (JP-A-2005-148508).
On the other hand, to increase the depth of field, many endoscope objective lenses have a large (i.e., dark) F number, in which case neither the spherical aberration nor the coma aberration is an important factor to determining image quality and the lateral color is a great factor to cause image quality deterioration. In particular, in recent years, with the density increase of solid-state imaging devices and the increase in the number of pixels, it has come to be necessary to correct for the lateral color sufficiently. To correct for the lateral color, it is preferable to dispose an optical member for the correction of lateral color at a position that is distant from the stop. In particular, on the image side of the stop, the optical member for the correction of lateral color becomes more effective as it is disposed closer to the image forming surface. However, in lens systems having a long back focal length, the optical member is not disposed close to the image forming surface and hence it is not easy to correct for the lateral color. Although the endoscope objective lens described in JP-A-2005-148508 has a sufficiently long back focal length, it still has room for improvement in terms of the correction of lateral color.